The following answer is taken from Wardale's response to Leonard Staghurst whose letter was published in Steam Railway #277 (copied below):
"Unfortunately Mr. Staghurst's comments betray his lack of knowledge, not only of what is possible with steam traction but also of what has already been achieved. Consider the following.
|Item||SAR 26 Class 4-8-4||5AT 4-6-0|
|Type of locomotive||Rebuild of 1950's design||New design|
|Quality of thermodynamic design||Restricted by structural limitations of existing design||State of the art|
|Number of cylinders||2||2|
|Simple or compound||Simple||Simple|
|Boiler pressure, lb. per sq. in.||225||305|
|Fuel||Coal||Gas oil or diesel fuel|
|Ash content of fuel||15%||0|
|Fuel calorific value Btu/lb.||12,000||18,400|
|Engine weight (excluding tender), metric tons||123||80|
|Maximum indicated h.p.||5060 (peak of measured i.h.p. curve)||3460 (calculated)|
|i.h.p. per ton of engine weight||41.1||43.3|
The above figures, which are amply supported by other data, show eloquently enough that the calculated power capacity of the 5AT is perfectly realistic, and I regard achieving it as a formality (and surpassing it a probability) - even on this planet! Likewise all factors concerning thermal efficiency, except the target combustion efficiency at maximum evaporation, which I have acknowledged will be a difficult problem. But that does not mean impossible, simply that very good engineering will be required to achieve it."